Emissions of pollutant chemicals have increased orders of magnitudes in the past 100 years due primarily to anthropogenic releases associated with industrial, agricultural, domestic, and recreational activity. Current research indicates that there are very strong correlations between the increase in these emissions and an overall increase in atmospheric temperatures (i.e. global warming) and an increased number of Category 4 and 5 hurricanes per annum. Furthermore, it is believed that ambient particulate matter in aerosol phase may include potentially toxic components. Researchers also believe that particulate matter and gases from industrial activities and vehicles may cause various health problems, such as asthma. These correlations between emissions of pollutant chemicals and the negative impact on environment and human health has led to more stringent worldwide emission standards for automobiles and other vehicles, as well as power plants, mines, and other industries.
In the United States, emission standards are set by the Environmental Protection Agency (EPA) as well as state governments (e.g. California Air Resource Board (CARB)). As of this writing, all new vehicles sold in the United States must meet the EPA's “Tier 1” emission standard. A more stringent standard, “Tier 2,” is being phased in for automobiles and should be completed by 2009. For diesel engines, on-road trucks and other vehicles will be required to meet more stringent standards by 2010 and off-road vehicles such as construction vehicles will be subject to Tier IV regulations. Accordingly, attaining ultra low emissions has become a top priority for combustion researchers as federal and state regulations continuously reduce the allowable levels of pollutants that can be discharged by engines, power plants, and other industrial processes.
In order to meet the emission standards of today and the future, researchers have made, and are continually striving to make, improvements to combustion engines, for example heavy duty diesel engines, gas combustion engines, power-generating gas turbines, and the like, and other emission sources. In addition to these developments, researchers are endeavoring for better methods and devices of measuring smaller particulate matter and low level gases and quantifying the chemical compositions of emissions.
Generally, chemical composition analysis of fine particulate matter, gases, and volatile and semi-volatile organic compounds from emissions sources consists of three major steps: (1) Representative conditioning and sampling; (2) Chemical analysis; and (3) Data analysis and explanation. The effective accuracies of Steps (2) and (3) are both dependent on step (1). For without an accurate and precise sampling procedure, no analysis of that sample could be said to represent valid data. Accordingly, without valid analysis, a full and complete explanation of the sample would not be available.
A conventional system for assessing particle mass and quantifying chemical composition of emission gases mixes emission gas with filtered air in a mixing chamber. The conventional system is illustrated in FIG. 1, and includes a sampling port 2 that feeds exhaust gases to a diluter 4, forming the mixing chamber, where the exhaust gases are diluted with the filtered air. The diluted gas mixture is then sampled by a sampling train 6. However, this system has many well recognized disadvantages. First, the partial/full/partial dilution sampling system in this conventional system would introduce more errors than a full/partial/full system. Second, the conventional system allows only for assessment of single type of compound. Accordingly, multiple sample runs are required to detect each of the chemical compounds necessary for a full compound assessment (particulate matter, volatile organics, semi-volatile organics, and gases, etc.) Furthermore, these measurements are made with different samples each time, and may add to inherent errors that are unavoidable to this system. These errors may lead to inaccurate measurements and quantification of data.
Work at the University of Wisconsin-Madison attempted to improve the conventional system. University of Wisconsin scientists used a device called an “augmented sampling system” to study the chemical composition and to assess particle size of diesel engine exhaust. See Chol-Burn Kweon, David E. Foster, James J. Schauer, and Shusuke Okada, “Detailed Chemical Composition and Particle Size Assessment of Diesel Engine Exhaust” SAE 2002-01-2670, Fall SAE Meeting 2002. The “augmented sampling system” disclosed by Kweon et al includes a secondary dilution tunnel for the diesel exhaust and a residence time chamber with radial sampling ports near the base of the residence time chamber. The secondary dilution tunnel of the augmented sampling system mixes filtered air with an emission gas sample without regard to temperature gradient between the surface of the dilution tunnel and the emission gas. This may lead to a high degree of particle loss and accordingly less accurate sampling due to thermophoresis.
Thermophoresis, or Ludwig-Soret effect, is thought to be related to Brownian movement biased by a temperature gradient. The thermophoretic force is a force that arises from asymmetrical interactions of a particle with the surrounding gas molecules due to a temperature gradient. Generally, a particle is repelled from a hotter surface and attracted to a cooler surface. Thus, as emission particles travel through a sampling system, cooler surface temperature of the system as compared to the emission gas would lead to greater thermophoretic force on the emission particles.
In the Kweon et al. augmented sampling system, the residence time chamber is heated to reduce thermophoresis. However, the heated residence time chamber is likely to fail in achieving realistic atmospheric conditions, as the addition of heat may underestimate the particulate matter emissions due to the reduced effects of nucleation and condensation and may also affect secondary reactions of volatile organic compounds and semi-volatile organic compounds and formation of secondary organic compounds.
A system that allows more accurate and precise sampling of emission products is needed, thereby contributing to better measurement and analysis of the emission products.